The present invention relates to a heat sensitive paper for thermal printing devices and a heat sensitive coating composition for such papers comprising a chromogenous basic triphenylmethane derivative, in particular a basic diarylphthalide derivative, and an acidic phenolic color-developer in a carrier composition.
The color-forming reaction between a substantially colorless chromogenous leucoform of a basic triphenyl methane dyestuff, in particular a chromogenous basic diphenylphthalane derivative, and an active acidic color developer, in particular a phenolic compound and/or an active inorganic solid such as clay or attapulgite, has been widely used in the field of duplication and printing processes, e.g. for the production of transfer and copying papers (see, e.g. U.S. Pat. Nos. 3,244,548 in the name of Sullivan et al. and 3,356,229 and 3,244,550 in the name of Farnham et al.).
Two principally different types of coating compositions for copying and transfer papers are known, namely pressure sensitive and heat sensitive coatings.
Pressure sensitive record materials are commonly surface coated with a composition wherein the chromogenous compound is included in microscopic pressure-rupturable capsules. These capsules consist of a nucleus of an oily liquid containing the chromogenous compound and a rugged shell which is impermeable to the oil and is formed by preparing an emulsion of the chromogen-containing oil in an aqueous solution of gum arabic and gelatin and curing the liquid film of gum arabic/gelatin surrounding the oil droplets with formaldehyde. A solid acidic color developing substance is included in the same coating composition yet outside the capsules or in a separate coating, if desired on a second paper, e.g. in the case of manifold material.
Upon locally applying printing or writing pressure to the material, the gum-arabic-gelatin-formaldehyde film will rupture, releasing the chromogenic liquid from the capsules and allowing the chromogen to come into contact with the active acidic color developer and to form a colored mark at the site of the applied pressure. Pressure sensitive record materials are disclosed, e.g. in U.S. Pat. Nos. 2,730,456 and 2,703,457. U.S. Pat. No. 3,920,510 discloses a pressure sensitive coating composition for copying paper, wherein the pressure-rupturable capsules contain a solution of a chromogenous fluoran compound in monoisopropyldiphenyl surrounded by a cured gum arabic-gelatin-formaldehyde film.
In the preparation of heat-sensitive compositions the formation of microcapsules is eliminated. Instead, the chromogenic compound and the phenolic color-developer both are distributed in finely divided solid form in a carrier or binder, in particular a polyvinylalcohol. Upon local application of heat, one of the color-forming reactants, usually the phenolic compound, is fluidized thus leading to an intimate contact between the two color-forming reactants and the taking place of the color-forming reaction localized at the site of heating at thermal printing temperatures. Heat-sensitive record materials containing a chromogenous and a color developing compound distributed in a polyvinyl alcohol coating composition are disclosed in U.S. Pat. No. 3,920,510, in U.S. Pat. No. 3,539,375 and in U.S. Pat. No. 3,674,535. Polyvinyl alcohol, which is a non-ionic water soluble polymer, is the most commonly used carrier-binder component in heat-sensitive coating compositions for thermal printing. Other additives may be added such as gum arabic which is disclosed in U.S. Pat. No. 4,168,845.
Conventional heat-sensitive coating compositions and papers for thermal printing coated therewith suffer from various disadvantages, such as a tendency for premature self-color development, lack of environmental stability, i.e. to heat, moisture and light, as well as an undesirable degree of pressure sensitivity of the coated paper. They also suffer from an inability to be coated on commodity grade paper without hinderence to performance. Cumbersome stock characteristics, that is, a tendency of the coated paper to not move freely during the heat-printing process and adhere to the printing head have been a problem. U.S. Pat. No. 4,168,845, the entire disclosure of which is herein incorporated by reference, describes an attempt to alleviate this problem by adding to the compositions a pigment with a specific oil absorption value.